Micromachined thermal mass flow sensors and insertion type flow meters and manufacture methods

Abstract

An integrated mass flow sensor is manufactured by a process of carrying out a micro-machining process on an N or P-type silicon substrate with orientation <100>. This mass flow sensor comprises a central thin-film heater and a pair of thin-film heat sensing elements, and a thermally isolated membrane for supporting the heater and the sensors out of contact with the substrate base. The mass flow sensor is arranged for integration on a same silicon substrate to form a one-dimensional or two-dimensional array in order to expand the dynamic measurement range. For each sensor, the thermally isolated membrane is formed by a process that includes a step of first depositing dielectric thin-film layers over the substrate and then performing a backside etching process on a bulk silicon with TMAH or KOH or carrying out a dry plasma etch until the bottom dielectric thin-film layer is exposed. Before backside etching the bulk silicon, rectangular openings are formed on the dielectric thin-film layers by applying a plasma etching to separate the area of heater and sensing elements from the rest of the membrane. This mass flow sensor is operated with two sets of circuits, a first circuit for measuring a flow rate in a first range of flow rates and a second circuit for measuring a flow rate in a second range of flow rates, to significantly increase range of flow rate measurements, while maintains a high degree of measurement accuracy.

1. An integrated mass flow rate sensor comprising:
 a set of temperature sensors connected as part of a first circuit with a heating element disposed between said temperature sensors for measuring a temperature difference between an upstream and downstream temperature sensing elements for measuring a flow rate in a first range of flow rates; and 
 a resistive sensing element comprising an ambient temperature sensor in series connection with a resistor Rc having a constant resistance in combination with said heating element as part of a second circuit for measuring a heat loss of said heating element with reference to an ambient temperature for measuring a flow rate in a second range of flow rates wherein said first circuit and said second circuit simultaneously apply both said temperature difference between said upstream and downstream temperature sensing elements and said heat loss of said heat element respectively for concurrently measuring said flow rate that may fall within said first range of flow rates or said second range of flow rates whereby said integrated flow rate sensor having an expanded range of flow rate measurement covering said first range and said second range. 
 
   
   
     2. The integrated mass flow rate sensor of  claim 1  wherein:
 said first circuit and said second circuit comprising respectively a first Wheatstone bridge circuit and a second Wheatstone bridge circuit simultaneously receive and apply both said temperature difference between said upstream and downstream temperature sensing elements and said heat loss of said heat element respectively for concurrently measuring said flow rate over said expanded range. 
 
   
   
     3. The integrated mass flow rate sensor of  claim 2  wherein:
 a ratio of a detectable maximum to minimum flow rates measurable by applying said first and second Wheatstone bridge circuits is about 1000:1. 
 
   
   
     4. The integrated mass flow rate sensor of  claim 2  wherein:
 said first Wheatstone bridge circuit and said second Wheatstone bridge circuit are further connected to a multiple channel analog to digital converter (ADC) for converting multiple analog signals into multiple digital signals for processing by a digital processing unit. 
 
   
   
     5. The integrated mass flow rate sensor of  claim 4  wherein:
 said digital processing unit further includes a data storage device for storing a digital signal versus flow rate calibration table for determining a flow rate measurement by using a digital signal from either said first Wheatstone bridge circuit and a corresponding signal from said ADC or said second Wheatstone bridge circuit and another corresponding signal from said ADC. 
 
   
   
     6. The integrated mass flow rate sensor of  claim 1  wherein:
 said set of temperature sensors and said heating element are disposed on a thermally isolated membrane extending over a hollow space underneath formed as a bulk-etched cavity in a silicon substrate. 
 
   
   
     7. The integrated mass flow rate sensor of  claim 6  wherein:
 said thermally isolated membrane is disposed on a top surface of said substrate and said hollow space under said membrane as said bulk-etched cavity is a cavity opened from a bottom surface opposite said top surface. 
 
   
   
     8. The integrated mass flow rate sensor of  claim 6  wherein:
 said thermally isolated membrane is disposed on a top surface of said substrate and said hollow space under said membrane as said bulk-etched cavity is a cavity opened from a bottom surface opposite said top surface along a <100> crystal plane. 
 
   
   
     9. The integrated mass flow rate sensor of  claim 6  wherein:
 said thermally isolated membrane is disposed on a top surface of said substrate and said hollow space under said membrane as said bulk-etched cavity is a TMAH bulk etching cavity from a bottom surface opposite said top surface. 
 
   
   
     10. The integrated mass flow rate sensor of  claim 6  wherein:
 said thermally isolated. membrane is disposed on a top surface of said substrate and said hollow space under said membrane as said bulk-etched cavity is a KOH bulk etching cavity from a bottom surface opposite said top surface. 
 
   
   
     11. The integrated mass flow rate sensor of  claim 6  wherein:
 said thermally isolated membrane is disposed on a top surface of said substrate and said hollow space under said membrane as said bulk-etched cavity is a plasma bulk etching cavity from a bottom surface opposite said top surface. 
 
   
   
     12. The integrated mass flow rate sensor of  claim 6  wherein:
 said temperature sensors and said heating element further comprising a Pt/Cr resistor. 
 
   
   
     13. An integrated mass flow rate sensor comprising:
 a set of temperature sensors with a heating element disposed between said temperature sensors connected as part of a first signal measuring circuit for measuring a temperature difference between said temperature sensors for measuring a flow rate in a first flow range; and 
 a resistive sensing element comprising an ambient temperature sensor in series connection with a resistor Rc having a constant resistance wherein said ambient temperature sensor and said resistor Rc connected in combination with said heating element as part of a second signal measuring circuit for measuring a mass flow rate by sensing a heat loss of said heating element wherein said first and second signal measuring circuits simultaneously measuring said temperature difference between said temperature sensors and said heat loss of said heating element respectively measure at least two different ranges of flow rate with a first range of flow velocity below a saturation flow velocity and a second range of flow velocity above said saturation flow velocity. 
 
   
   
     14. The integrated mass flow rate sensor of  claim 13  wherein:
 said integrated mass flow rate sensor simultaneously generating two analog signals representing respectively said temperature difference between said set of temperature sensors for measuring a flow rate in a first range of flow velocity below said saturation flow velocity and said heat loss of said heating element for measuring a flow rate in a second range of flow velocity above said saturation flow velocity. 
 
   
   
     15. The integrated mass flow rate sensor of  claim 13  further comprising:
 a two-channel analog to digital converter (ADC) for simultaneously converting said two analog signals to two digital signals representing digital signals for measuring said flow rate that may fall within said first or second ranges of flow rates. 
 
   
   
     16. The integrated mass flow rate sensor of  claim 13  wherein:
 said two temperature sensing signal measuring circuits comprising a first Wheatstone bridge circuit and a second Wheatstone bridge circuit. 
 
   
   
     17. The integrated mass flow rate sensor of  claim 16  wherein:
 a ratio of a detectable maximum to minimum flow rates measurable by applying said first and second Wheatstone bridge circuits is about 1.000:1. 
 
   
   
     18. The integrated mass flow rate sensor of  claim 16  wherein:
 said first Wheatstone bridge circuit and said second Wheatstone bridge circuit are further connected to a multiple channel analog to digital converter (ADC) for converting multiple analog signals into multiple digital signals for processing by a digital processing unit. 
 
   
   
     19. The integrated mass flow rate sensor of  claim 18  wherein:
 said digital processing unit further includes a data storage device for storing a digital signal versus flow rate calibration table for determining a flow rate measurement by using a digital signal from either said first Wheatstone bridge circuit and a corresponding signal from said ADC or said second Wheatstone bridge circuit and another corresponding signal from said ADC. 
 
   
   
     20. The integrated mass flow rate sensor of  claim 13  wherein:
 said set of temperature sensors and said heating element are disposed on a thermally isolated membrane extending over a hollow space underneath formed as a bulk-etched cavity in a silicon substrate. 
 
   
   
     21. The integrated mass flow rate sensor of  claim 20  wherein:
 said thermally isolated membrane is disposed on a top surface of said substrate and said hollow space under said membrane as said bulk-etched cavity is a cavity opened from a bottom surface opposite said top surface. 
 
   
   
     22. The integrated mass flow rate sensor of  claim 20  wherein:
 said thermally isolated membrane is disposed on a top surface of said substrate and said hollow space under said membrane as said bulk-etched cavity is a cavity opened from a bottom surface opposite said top surface along a <100> crystal plane. 
 
   
   
     23. The integrated mass flow rate sensor of  claim 20  wherein:
 said thermally isolated membrane is disposed on a top surface of said substrate and said hollow space under said membrane as said bulk-etched cavity is a TMAH bulk etching cavity from a bottom surface opposite said top surface. 
 
   
   
     24. The integrated mass flow rate sensor of  claim 20  wherein:
 said thermally isolated membrane is disposed on a top surface of said substrate and said hollow space under said membrane as said bulk-etched cavity is a KOH bulk etching cavity from a bottom surface opposite said top surface. 
 
   
   
     25. The integrated mass flow rate sensor of  claim 20  wherein:
 said thermally isolated membrane is disposed on a top surface of said substrate and said hollow space under said membrane as said bulk-etched cavity is plasma bulk etching cavity from a bottom surface opposite said top surface. 
 
   
   
     26. The integrated mass flow rate sensor of  claim 13  wherein:
 said temperature sensors and said heating element further comprising a Pt/Cr resistor. 
 
   
   
     27. The integrated mass flow rate sensor of  claim 13  wherein:
 said ambient temperature sensor is connected in series with a resistor having substantially a constant resistance disposed on a silicon substrate and further disposed on a thermally isolated membrane extending over a hollow space underneath as a bulk-etched cavity-supported on a carrier substrate. 
 
   
   
     28. The integrated mass flow rate sensor of  claim 27  further comprising:
 a heat sink disposed on said cater substrate attached to said temperature sensors and said heater and said ambient temperature sensor for dissipating heat generated therefrom. 
 
   
   
     29. The integrated mass flow rate sensor of  claim 27  further comprising:
 bonding wires for connecting said temperature sensors and said heating element and said ambient temperature sensor to said signal measuring circuits through said carrier substrate. 
 
   
   
     30. The integrated mass flow rate sensor of  claim 29  further comprising:
 a corrosion free encapsulating and flow conditioning tube having structural strength to sustain high flow pressures encapsulating said integrate mass flow rate sensor. 
 
   
   
     31. The integrated mass flow rate sensor of  claim 29  further comprising:
 a feed-through connector for connecting said integrated mass flow rate sensor disposed in a flow channel enclosed in a flow pipe to a flow-meter processor disposed outside of said flow pipe wherein said flow meter processor further receives signals from said signal measuring circuits. 
 
   
   
     32. A method for measuring a mass flow rate comprising:
 measuring a flow rate in a first range of flow rates by connecting a set of temperature sensors as part of a first circuit and disposing a heating element between said set of temperature sensors for measuring a temperature difference between upstream and. downstream temperature sensing elements; and 
 measuring a flow rate in a second range of flow rates by connecting a resistive sensing element to function as an ambient temperature sensor connected in series with a resistor Rc having a constant resistance as part of a second circuit for measuring a heat loss of said heating element with reference to an ambient temperature wherein said first circuit and said second circuit simultaneously apply both said temperature difference between said upstream and downstream temperature sensing elements and said heat loss of said heat element respectively for concurrently measuring said flow rate that may fall within said first range of flow rates or said second range of flow rates. 
 
   
   
     33. The method of  claim 32  further comprising:
 receiving measurement signals of said first flow rate and said second flow rate as analog signals into a two-channel analog to digital converter (ADC) for simultaneously converting said two measurement signals to two digital signals representing digital signals for measuring said flow rate that may fall within said first or second ranges of flow rates. 
 
   
   
     34. The method of  claim 32  wherein:
 said step of connecting said set of temperature sensors and said pair of heating element as part of a first circuit and disposing a heating element between said set of temperature sensors further comprising a step of disposing said set of temperature sensors and said heating element on a thermally isolated membrane extending over a hollow space underneath as a bulk-etched cavity in a substrate.